Did your referee instructor tell you not to call fouls for U8 games? Well most of the time that is true, but when the play gets rough, and verbal warnings fail, you should call fouls.
Fouls are hard in 2nd grade, because many fouls require a direct free kick, and for a game without goalies, that is almost always an automatic goal. SO a good rule of thumb is to think about the infringement in terms of whether it is really worth a score. So even if a 2nd grader is pushing (which calls for a direct free kick) you would probably want to call it as something more minor, like dangerous play, which gets an indirect free kick.
You can actually say that to a player – “you are pushing, but we will call it dangerous play so that it is an indirect kick instead of a direct kick – next time you do it, I will call it what it is and the other team will get a goal.” Of course, if the player is walking around shoving kids to the ground and tripping them malevolently, then by all means give the opponent a direct free kick. And for really violent play, use a yellow card.
Before a U8 game, you should remind yourself of the rules for fouls and have a general knowledge of how to line the kids up for the kicks:
list of fouls: http://www.bays.org/node/31
free kicks: http://www.bays.org/node/32
“Direct" means that a goal can be scored directly against the defending team; "Indirect" means that a goal cannot be scored unless the ball has been touched by any player in addition to the kicker before entering the goal.
One would hope that a few choice words and an indirect free kick or two would be enough for any 2nd grade game... I think that is what the instructor who told you not to call fouls really meant. But when all hope fails, please be sure that you know the rules, so that you can make the calls.